Check cached sources with upstream after the cache TTL (closes #69)

A cached image was served until eviction, so a change at the same URL
was never picked up.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it, and older variants are converted
again. A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified, so a variant
whose source was evicted is checked the same way.

A variant file's modification time is its source's check time when it
was made or renewed, so a hit reads no database row.

Model: opus-5-5

internal/imageproxy/stats_internal_test.go

@@ -94,7 +94,7 @@ type fakeUpstream struct {
 }
 
 func (u *fakeUpstream) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	return &httpfetcher.FetchResult{
 		Content:       io.NopCloser(u),